Re: [PATCH] implement privmode support in dash

[jca+dash@xxxxxxxxxx (Jérémie Courrèges-Anglas)]

Tavis Ormandy <taviso@xxxxxxxxxx> writes:

> On Thu, Aug 22, 2013 at 1:35 PM, Jilles Tjoelker <jilles@xxxxxxxx> wrote:
>> I think there is no reason to deviate from other shells here. Therefore,
>> please call it "privileged".
>>
>
> Agreed.
>
>>> In bash and FBSD, after starting with -p, set +p can be used to drop
>>> privileges. With your patch, dash accepts set +p, but silently ignores it.
>>
>>> How does something like the attached, to be applied on top of your
>>> patch, look?
>>
>>> [snip]
>>> +     if (!on && (uid != geteuid() || gid != getegid())) {
>>> +             setuid(uid);
>>> +             setgid(gid);
>>> +             /* PS1 might need to be changed accordingly. */
>>> +             choose_ps1();
>>> +     }
>>> +}
>>
>> This code tries to use setuid() and setgid() to drop all privilege,
>> which is only correct if the privilege to be dropped is UID 0, or on BSD
>> systems. It would be better to use setresuid() or setreuid(), and change
>> the GID before changing the UID.
>
> This is logic duplicated from pdksh and bash, I'm slightly reluctant
> to do things differently, unless it's not going to get committed
> otherwise.

pdksh is only maintained by OpenBSD, afaik (mksh syncs regularly).
The current code rather looks like this:

	if (f == FPRIVILEGED && oldval && !newval) {
		gid_t gid = getgid();

		setresgid(gid, gid, gid);
		setgroups(1, &gid);
		setresuid(ksheuid, ksheuid, ksheuid);
	} ...

> You can see some code snippets here:
> http://blog.cmpxchg8b.com/2013/08/security-debianisms.html
>
>> Apart from that, it is better to check the return value from setuid()
>> and similar functions. In particular, some versions of Linux may fail
>> setuid() for [EAGAIN], leaving the process running with the same
>> privileges.
>
> I don't think this is true anymore, but I have no strong objection to
> adding it, so long as it's noted that bash and pdksh do not do this.
>
> Tavis.

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